1. Field of the Invention
The present invention relates to an induction heater used in a fixing unit of an image forming apparatus that fixes a toner image produced on a recording sheet by means of heating and, more particularly, to an induction heater using an electromagnetic induction technique (an IH technique) as a heating technique.
2. Description of the Related Art
There is recently a growing demand for energy conservation and speedup of an image forming apparatus, such as a printer, a copier, and a facsimile. In order to attain the required performance, an improvement in heat efficiency of the fixing unit employed in the image forming apparatus is important.
A proposed technique is for causing a fixing unit of an electromagnetic induction heating type to generate Joule heat from an eddy current that has developed in a magnetic metal member from an alternating field, thereby letting a heating element including a metallic member effect electromagnetic induction heating (JP-A-2003-223063). However, the image forming apparatus has encountered a problem about indefinite sizes of sheets; namely, a necessity for coping with a plurality of widths of sheets.
In order to address the problem, there is another proposed fixing unit of electromagnetic induction heating type including an exciting coil and a sub-induction coil (JP-A-2009-128551). The exciting coil and the sub-induction coil located inside the exciting coil are produced within a single plane. When a circuit is closed by a switch, to thus become short-circuited, the sub-induction coil described in connection with JP-A-2009-128551 is electromagnetically coupled with an exciting coil by way of electric capacity changeover means. When the switch is conversely opened, to thus open the circuit, the sub-induction coil and the exciting coil are brought into an electromagnetically uncoupled state.
Likewise, in order to address various widths of recording sheets, there is available still another proposed fixing unit of electromagnetic induction heating type that cancels magnetic flux by piling stepwise a degaussing coil on an exciting coil (JP-A-2008-139475). In JP-A-2008-139475, an exciting coil is wound around the fixing unit along a fixing roller 1, and a first degaussing coil is placed on the exciting coil. Further, a second degaussing coil is piled on the first degaussing coil in stacked manner. The exciting coil and the degaussing coil are set to the same width.
Incidentally, in a turn portion of an exciting coil of the electromagnetic induction heating type, a temperature drop is likely to arise in the distribution of heat of a heating roller. For this reason, there is also available a proposed fixing unit that prevents occurrence of a temperature drop in the turn portion by providing the turn portion with a magnetic flux focusing member (JP-A-2005-235637). There is used an annular exciting coil including parallel portions that extend in parallel with a heating roller along its longitudinal direction and turn portions that are provided at both ends of the parallel portions. A magnetic flux focusing member is provided at an area of the turn portion where orientations of magnetic fields developing from the turn portion are aligned, thereby preventing occurrence of a temperature drop at an interior of the turn portion. Temperature uniformity of the heating member achieved in its longitudinal direction is thereby enhanced.
According to a paper width control method that is a current mainstream, a degaussing coil capable of addressing various widths of recording sheets is provided on an exciting coil. The degaussing coil is short-circuited in accordance with the size of a recording sheet, thereby cancelling magnetic fluxes of the exciting coils and preventing occurrence of a temperature rise in a non-sheet-feeding area. Meanwhile, an ever-increasing demand recently exists for speedup of the image forming apparatus, and heat capacity of the fixing unit for shortening a warm-up period is reduced year by year. A reduction in heat capacity means that a heating roller is configured so as to become easily heated. Enhancement of the capability to suppress an increase in the temperature of an unused non-sheet-feeding area (i.e., temperature rise controlling capability) has therefore been sought.
However, the fixing unit of electromagnetic induction heating type described in connection with JP-A-2009-128551 has a structure in which the sub-induction coil is coupled to the exciting coil by way of the electric capacity changeover means. The sub-induction coil is placed inside of the exciting coil that is provided in the same plane for controlling magnetic fluxes. A gap is likely to develop between the sub-induction coil and the exciting coil. Even when switching is carried out by the electric capacitance changeover means, magnetic fluxes are not sufficiently canceled, which in turn raises a problem of remaining of some magnetic fluxes. Specifically, under the electromagnetic induction heating method, a temperature rise in the non-sheet-feeding area cannot sufficiently be controlled. Moreover, there is another problem of a necessary quantity of heat becoming conversely deficient in the neighborhoods of both ends of a sheet feeding area. Specifically, a desirable temperature distribution that exhibits a uniform temperature only in the sheet feeding area is not attained. On the other hand, an attained temperature distribution is gradual such that a temperature gradually decreases with a closer approach toward both ends of the sheet feeding area and that the temperature drop further increases with an increasing approach toward the non-sheet-feeding area.
In the fixing unit of electromagnetic induction heading type described in connection with JP-A-2008-139475, a degaussing coil is piled on an exciting coil, and recording sheets of various widths are fixed. However, this type also requires to stack a degaussing coil on an exciting coil in the form of layers, and a gap is likely to develop between the exciting coil and the degaussing coil. Cancellation of the magnetic fluxes performed by the degaussing coil is insufficient, so that some of the magnetic fluxes still remain. Even the fixing unit of this type cannot sufficiently control an increase in the temperature of the non-sheet-feeding area, and heat capacity is likely to become deficient at both ends of the sheet feeding area. Specifically, the temperature distribution does not become uniform in the sheet feeding area but exhibits a decline at each end of the sheet feeding area instead.